In spectrophotometry, radiation from a source passes through a sample cell to a photodetector which measures the amount of radiation absorbed by the sample fluid in the cell. The output of the detector is a measure of absorbance at a particular wavelength of radiation. The quantitative presence of certain materials in the sample is identified by particular wavelengths characteristically absorbed by the materials. An important use of spectrophotometric detectors is in chromatography wherein the components of a chromatographic column are separated in a column and the radiation absorbance of the separated components are then measured by a spectrophotometric detector.
In such detectors, radiation transparent optical windows allow radiation from the source to pass through the cell to the detector. In a common spectrophotometric detector, radiation passes through an entrance window, through the cell in a direction parallel to the flow of sample fluid through the cell, and through an exit window to the detector. Flat windows or plano convex lenses typically have been used. U.S. Pat. No. 4,192,614 to deMey et al. shows a detector assembly with flat windows at the entrance and exit openings in the cell. A lens focuses the radiation in a pattern which converges in the cell.
Another type of commonly used detector has divergent optics with sample fluid flow across a substantially planar radiation field in the cell. Such crossflow cells are typified by the Milton Roy LDC microcell used in conjunction with the LDC Model 1204D spectoMonitor detector. Crossflow cells permit close coupling of the cell to the outlet end of a chromatographic column.
Another type of crossflow cell is available from Guided Wave Inc. of California. The Guided Wave cell comprises a standard 1/4 inch cross union adapted to receive opposing fiber optic transmission probes. The probes contain an external sapphire window sealed into a 1/4 inch metal tube. The metal tube also contains a suitable collimating lens. The tube which can be made from various materials including 316 stainless steel, Monel 400 or Hastelloy C276 alloys is sealed to the sapphire window with a soft glass frit. The glass frit is fused to prevent leaks between the sapphire window and the inner wall of the 1/4 inch tube. An additional epoxy seal is used to coat the outer surface of the fused soft glass seal.
While the Guided Wave flow cell can be used in many services, it cannot be used in a hydrogen fluoride atmosphere or in a hydrogen fluoride, hydrogen chloride, chlorine atmosphere or in the presence of a strong caustic or in any other service where the epoxy and glass seal would be attacked by the materials flowing through the cell.
It is desirable to provide a cross union flow cell which can be used in such corrosive environments and in particular, in such environments at elevated temperatures.